Collect the hidden icon in the listed location to unlock the corresponding mini-game:

Aeronauts: Secret room in level 4.
Apple Masher: Secret room on top of the haystacks in level 3.

Bale Out: Secret room in Zuckerman's Barn.
Bounce: Enter the hole in the tree and search in the branches in level 8.
Bumper Cars: Ledge outside the circus tent in level 13.

Food Catch: Enter the tires and look on the tire pile in level 10.

Ring Toss: Secret room outside the circus tent in level 14.

Snacktime for Templeton: Enter the tires and look on the tire pile in level 9.
Spree Ball: On top of the Information Booth in level 14.

Water Fun: Enter the hole in the tree and search the branches in level 5.
Webbing Crashers: Secret room next to the water wheel in level 6.